Insulated electrical-conductors, such as wire and cable, are usually subjected to a high voltage test prior to use as a check against any unseen imperfections which may be present in the insulation. This test may be made either as the insulation is extruded onto the conductor or may be performed in some subsequent operation. Generally, the insulated conductor, which is suitably grounded, is a passed through an electrode of some type which applies a high voltage to the outer surface of the insulation. When a hole or void is present in the insulation, an arc is generated between the electrode and the grounded conductor to provide an insulation fault signal.
A testing apparatus of this type is disclosed in U.S. Pat. No. 5,132,629 issued to Clinton wherein a test station includes the high AC voltage source and an electrode for applying the high test voltage between the outer surface of the insulation and the conductor. The test station is coupled to a remote control unit which provides a fault detection circuit for distinguishing between corona current generated by corona effects and arc current caused by a fault in the insulation of the insulated conductor.
In the AC testing of insulated wire or cable, it is desirable to be able to detect the current produced by the smallest possible arc from the electrode to the conductor through a pin hole, crevice or other imperfection in the insulation.
Additionally, it is also desirable to prevent the current due to normal corona effects which occur in the air spaces between the elements of the electrode and the insulation surface from producing erroneous indications of an insulation fault condition by the detecting means.
It is further often desirable that the control unit or fault detecting means in such testing apparatus be separate and remotely located from the testing station and the high voltage transformer used to excite the electrode by distances of up to 100 meters or more without substantially degrading the performance of the apparatus.
It is a general aim therefore of the present invention to prevent normal corona effects from erroneously being detected as an insulation fault by enhancing the sensitivity of the test apparatus to discriminate between the current of a low current fault arc and normal corona current.
It is a further aim of the present invention to be able to locate the fault detection means at some distance from the high voltage transformer without degrading the detection characteristics of the fault detection means.